In sheet-fed offset printing, a printed control strip comprising information for measuring the quality of the printed sheets is often added to the printed sheets along the front sheet edge. These control strips comprise measuring fields which are sensed photoelectrically (for example, densitometrically or colorimetrically), and converted to corresponding measurement data values. The obtained data values are subsequently compared against predetermined ideal values, thus providing control data for making printing corrections such as color control adjustments. Additionally, the print control strip can be utilized for obtaining processing information, such as the rate of press operation.
The photoelectric sensing measurements of these measuring fields are sometimes obtained with a manually guided measuring head, for example, a hand-operated densitometer. However, when a substantial number of measuring fields are present in the print control strip, motorized systems have been developed to transversely drive the measuring heads (i.e., densitometers and/or colorimeters) over the print control strip in order to more rapidly obtain the measurements. Such devices are described in U.S. Pat. Nos. 4,200,932 and 4,671,661, (corresponding to German Patent No. 2,728,738 and European Patent No. 0,149,424 A2, respectively), and in European Patent No. 0,228,347 B1.
However, these aforementioned devices require an attendant to first extract sample sheets containing the control strip out of a stack of printed sheets delivered by the printing machine. The attendant must carefully align the sheet upon a support surface (i.e., control desk) beneath the corresponding measuring device, and then activate the motorized measuring device. Thus, the measuring operation consumes a substantial amount of the attendant's time, preventing performance of other duties, including supervising and other monitoring functions.
U.S. Pat. No. 5 029,525 (corresponding to German Patent No. DE 3,612,067 A1) is directed to a system wherein specimen-sheets are automatically extracted and conveyed directly to an adjacent control desk for measurement with a traversing densitometer. However, with this system the control desk for performing the measurements must be located adjacent the printing machine, in the direction of sheet conveyance. Accordingly, accessibility to the delivery stack is significantly reduced because of the location of the control desk, and thus to remove the stack of sheets requires either that the sheets be removed laterally or else that the control desk be temporarily displaced. Additionally, the control desk increases the overall length of the printing machine. Finally, a device for removing the measured sheets must be provided so that these specimen sheets do not accumulate after the measurement procedure.
Alternatively, to rapidly obtain the measured values of a print control strip without the need for an attendant, photoelectric sensing devices are disposed directly in the printing machine. For example, U.S. Pat. No. 5,095,818 (corresponding to German Patent No. DE 3,034,212 C2), DD Patent Specification 158,758 and DE 3,930,782 C1 describe printing machines having this "in-line" sensing feature. However, since at higher printing speeds the measurements must be obtained extremely rapidly, these machines require a multiplicity of photoelectric sensing devices in order to scan the large number of measuring fields distributed over the format width of the sheet. Accordingly, such systems are relatively expensive and, for reasons of space, require complex scanning arrangements.
Meanwhile, U.S. Pat. No. 4,730,822 is directed to a device for stacking certain sheets on a delivery table of a printing machine in a manner offset from the other sheets in the stack. In other words, certain sheets are placed onto the stack so that an edge of the sheet sticks out from the rest of the stack. However, this device is not used for obtaining print quality measurements, but only to designate certain portions of stacks by means of the overhanging sheets, i.e., like a bookmark.